COBALT(II) HYDROXIDE 243

COBALT(II) HYDROXIDE

[21041-93-0]

Formula: Co(OH)2; MW 92.95

Synonyms: cobaltous hydroxide; cobaltous hydrate

Uses

Cobalt(II) hydroxide is used as a drier for paints and varnishes and is added to lithographic printing inks to enhance their drying properties. Other applications are in the preparation of cobalt salts; as a catalyst; and in storage battery electrodes.

Physical Properties

Two forms occur, a rose-red powder (more stable) and a bluish-green powder less stable than the red form; rhombohedral crystals; density 3.597 g/cm3; decomposes on heating; practically insoluble in water 3.2 mg/L; Ksp 1.0x10–15; soluble in acids and ammonia; insoluble in dilute alkalis.

Thermochemical Properties

Preparation

Cobalt(II) hydroxide is obtained as a precipitate when an alkaline hydroxide is added to an aqueous solution of cobalt(II) salt:

CoCl2 + 2NaOH → Co(OH)2 + 2NaCl

Co(NO3)2 + 2NaOH → Co(OH)2 + 2NaNO3

Reactions

Thermal decomposition to cobaltous oxide, CoO, occurs at 168°C in a vacuum.

Cobalt(II) hydroxide is oxidized by air and other oxidizing agents, forming cobalt(III) hydroxide, Co(OH)3. Reactions with mineral acids produce corresponding Co2+ salts.

Analysis

Elemental composition: Co 63.40%, H 2.17%, O 34.43%. Cobalt(II) hydroxide is dissolved in nitric acid and the acid extract is analyzed for cobalt metal by AA, ICP or other instrumental techniques following appropriate dilution (see Cobalt).

244 COBALT(II) IODIDE

COBALT(II) IODIDE

[15238-00-3]

Formula: CoI2; MW 312.74; also forms a hexahydrate, CoI2•6H2O, MW 420.83 Synonyms: cobaltous iodide; cobalt diiodide

Uses

Cobalt(II) iodide is used for analysing water in organic solvents; and as a color indicator to determine moisture and humidity.

Physical Properties

Exists in two isomorphous forms, α– and ß–forms; both modifications highly hygroscopic. The α–form is black hexagonal crystal; density 5.58 g/cm3; turns dark green in air; melts at 560°C; disolves in water giving pink coloration. The α–forms sublimes in vacuo, partly forming an isomorous yellow modification–the anhydrous β–form.

The β–modification is a yellow powder; density 5.45 g/cm3; converts to the α–form when heated to 400°C; absorbs moisture from air, the yellow powder becoming green droplets; dissolves readily in water forming a colorless solution which turns pink on heating.

The hexahydrate is red hexagonal crystals; density 2.90 g/cm3; loses water at 130°C giving anhydrous iodide; soluble in water, ethanol, acetone, chloroform and ether, forming colored solutions, (while the aqueous solution is red below 20°C and green above this temperature; the salt forms blue solution in ethanol, chloroform and ether).

Thermochemical Properties

Preparation

Cobalt(II) iodide is prepared by heating cobalt powder in a stream of hydrogen iodide at 400 to 450°C:

The product obtained is the black crystalline α–form.

Cobalt(II) iodide also may be made by heating cobalt powder with iodine vapor.

Analysis

Elemental composition: Co 18.84%, I 81.16%. CoI2 may be identified from its varying colors in different solvents. Under varying conditions, its aqueous solution may be analyzed for cobalt by AA, ICP or other instrumental techniques after appropriate dilution (see Cobalt). Iodide anion may be analyzed in sufficiently diluted aqueous phase by ion chromatography. Also, the analy-

COBALT(II) NITRATE 245

sis of the compound dissolved in chloroform or acetone at low ppm concentration may be performed by GC/MS. The presence of characteristic iodide anion mass, 127 amu, in the mass spectra serves as a further confirmatory test.

COBALT(II) NITRATE

[10141-05-6]

Formula: Co(NO3)2; MW 182.94; occurs in common hexahydrate form, Co(NO3)2•6H2O [10026-26-9], MW 291.03

Uses

Cobalt nitrate is used in the decoration of porcelain and stones; in the manufacture of invisible inks and cobalt pigments; in hair dyes; in animal feeds; as an additive to soils; in catalysts preparation; and in vitamin supplements.

Physical Properties

The hexahydrate is red monoclinic crystal; deliquescent in moist air; density 1.87 g/cm3; decomposes at 55°C, losing three molecules of water; decomposes to green cobalt(II) oxide on further heating at 74°C; very soluble in water (134 g/100mL at 0°C); also soluble in alcohols and acetone.

The anhydrous salt is pale red powder; density 2.49 g/cm3; decomposes around 100°C; soluble in water.

Preparation

Cobalt(II) nitrate is prepared by treating the metal, or its oxide, hydroxide or carbonate with dilute nitric acid. The solution on evaporation yields red crystals of hexahydrate:

Co + 2HNO3 → Co(NO3)2 + H2

Co(OH)2 + 2HNO3 → Co(NO3)2 + 2H2O

CoCO3 + 2HNO3 → Co(NO3)2 + CO2 + H2O

Analysis

Elemental composition (anhydrous salt): Co 32.33%, N 15.31%, O 52.47%. The aqueous solution may be analyzed for cobalt by AA or ICP or other instrumental methods. The nitrate anion may be measured by ion chromatography or nitrate ion-selective electrode. The solutions may require sufficient dilution for all these measurements.

246 COBALT OCTACARBONYL

Toxicity

The compound is toxic by oral, subcutaneous, and intravenous routes. LDLO oral (rabbit): 250 mg/kg

LDLO subcutaneous (rabbit): 75 mg/kg

COBALT OCTACARBONYL

[10210-68-1]

Formula: Co2(CO)8; MW 341.95

Synonyms: dicobalt octacarbonyl; cobalt carbonyl; cobalt tetracarbonyl dimer

Uses

Cobalt octacarbonyl is used as a catalyst in the Oxo process (see Carbon Monoxide). It also is used as a catalyst for hydrogenation, isomerization, hydrosilation and polymerization reactions. The compound is also a source of producing pure cobalt metal and its purified salts.

Physical Properties

Orange crystals; density 1.78 g/cm3; melts at 5l°C; decomposes above this temperature; insoluble in water; soluble in most organic solvents including alcohol, ether, carbon disulfide.

Preparation

Cobalt octacarbonyl is prepared by the reaction of finely divided cobalt with carbon monoxide under pressure:

2Co + 8CO → Co2(CO)8

The compound may be prepared in a similar way from cobalt(II) iodide. Also, it may be prepared by thermal decomposition of cobalt carbonyl hydride:

Reactions

Cobalt octacarbonyl forms complexes with many types of ligands, replacing one or more CO groups.

Reaction with potasium cyanide forms a cyano derivative that probably has the structure K3[Co(CN)5(CO)].

Reaction with ammonia forms ammine salt, [Co(NH3)6][Co(CO)4]2 liberating carbon monoxide (Hieber, W. and H. Schulten. 1937. Z. Anorg. Allgem. Chem., 236, p. 17). In a strongly alkaline solution, cobalt octacarbonyl undergoes hydrolysis, forming cobalt carbonyl hydride. This hydride, used in organic synthesis as a catalyst, may be prepared in a solution of hexane or toluene by adding octacarbonyl to dimethylformamide (DMF), followed by acidification: